U.S. Pat. No. 4,645,979 to the applicant describes a display device consisting of a discharge lamp for which the size of the illumination region can be dynamically controlled in order to create a "moving" or "sweeping" effect. The discharge lamp is comprised of an elongated glass tube containing an inert gas such as neon. The discharge lamp has a first electrode internal to the tube and a second electrode in the form of a conductive film on the exterior surface of the tube. A conductive film of tin oxide is produced by spraying an aqueous solution of tin halide onto the exterior surface of the tube at a temperature of 500 to 700 degrees Celsius.
The sweeping effect is produced by applying a sweeping signal to the electrodes of the discharge lamp, wherein the sweeping signal is a periodic sequence of pulses of increasing amplitude. As the pulses increase in amplitude the illumination region of the discharge lamp increases in size. The sweeping display produced is useful, for example, for conveying information pertaining to motion or direction, or for drawing attention to the a textual display formed by multiple discharge lamps.
In order to generate the sweeping signal, a pulse generator is used to generate a pulse signal in the range of 300 Hz to 20 kHz, and a sawtooth waveform generator is used to vary the amplitude of the pulses at a relatively slow rate. The resulting periodic sequence of pulses of increasing amplitude is applied to the primary winding of a step-up transformer. The step-up transformer boosts the amplitude of the pulses to the range of 500 to 1200 volts, and the resulting signal is applied to the electrodes of the discharge lamp.
This art prior system requires a variable voltage source such as a sawtooth waveform generator to vary the pulse amplitude. A microprocessor can be used to generate the variable voltage waveform, but a digital-to-analog converter is generally needed to convert the binary output of the microprocessor into a corresponding voltage waveform.
An objective of the invention, therefore, is to eliminate the need for a variable voltage source, and to thereby reduce the cost and complexity of the circuit for generating the sweeping signal. A further objective of the invention is to produce a sweeping signal capable of generation using ordinary logic-level components. A further objective of the invention is to provide two alternative embodiments of the exterior electrode of the discharge lamp.